1. Field of the Invention
The present invention relates to a semiconductor device having a structure in which a first layer of a group-III-V compound semiconductor and a second layer of a group-IV element semiconductor are laminated, and to a process of producing such a semiconductor device.
The herein-used term "group-III-V compound" means a compound of elements from the groups III and V of the periodic table and "group-IV element" means an element from the group IV of the periodic table.
2. Description of the Related Art
It is known that semiconductor devices such as high speed switching devices are produced by using crystal growing processes such as Molecular Beam Epitaxy (MBE), Metalorganic Chemical Vapor Deposition (MOCVD) and Metalorganic Vapor Phase Epitaxy (MOVPE), in which a semiconductor layer having a desired energy gap is grown from a gas phase source epitaxially on a semiconductor substrate heated at a temperature favorable for the growth, i.e., about 600.degree. to 700.degree. C. for example
FIG. 1 shows a semiconductor wafer having a typical lamination structure obtained by such a process and composed of a semi-insulating substrate 11 of GaAs, a buffer layer 12 of non-dope GaAs, a carrier-transporting channel layer 13 of non-dope Ge, and an active layer 14 of p-GaAs, the latter three layers having been grown in that order on the substrate 11.
To produce a semiconductor device such as a high speed switching device, the wafer of FIG. 1 is treated by fine processing, during which the shown structure is exposed to high temperatures of about 600.degree. to 700.degree. C. for example.
However, there is a problem in that, during the growth and fine processing steps, component elements of the layers 12, 13 and 14, when having a large diffusion coefficient in the neighboring layers, mutually diffuse from one to the other of the neighboring layers to form an extensive mutual diffusion zone on or near the interface between the neighboring layers, as depicted by the numerous dots in FIG. 1. The resulting extensive mutual diffusion zone, particularly when formed in the carrier-transporting layer 13, degrades the purity of the crystal of the layer 13 and the electric property of the product semiconductor devices such as a high speed switching device.
Japanese Unexamined Patent Publication (Kokai) No. 2-271670 proposed a Ge-GaAs heterojunction having a mutual diffusion-preventing layer referred to as a "transitional layer" or an "intermediate compound semiconductor layer" interposed between the GaAs and Ge semiconductor layers; the "transitional" and "intermediate" layers contain a metal and an element, respectively, having a greater force of bonding with Ga, As and Ge than the bonding forces between Ge and Ga and between Ge and As; and the disclosed advantageous substances are Al and Al-intermetallic compounds for the "transitional" layer and AlAs and GaAlAs for the "intermediate" layer. JPP'677 also disclosed a process of forming such a heterojunction, in which the Ge layer, the "transitional" or "intermediate" layer, and GaAs layer are formed in that order or vice versa without exposure to air.
JPP'670, however, does not consider the lattice coherency of the interposed layer with the neighboring compound (GaAs) or element (Ge) semiconductor layers. For example, the Example portion states that a lower semiconductor layer is an "epitaxial" layer, but does not refer to the epitaxy of the interposed layer and the upper semiconductor layer with respect to the underlying layers. A good electrical property cannot be obtained when a device has a structure in which the lattice coherency of the interposed layer with both the upper and lower semiconductor layers is not established.
JPP'670 also does not consider the actual feasibility of a process of forming the interposed layer. For example, although JPP'670 in the Example proposed Al as an advantageous substance of the interposed, mutual diffusion-preventing layer, the present inventors have found that Al usually forms discrete clusters on a substrate and cannot form a continuous layer with a thickness as small as several atomic layers.
JPP'670 also fails to consider whether the proposed interposed layer can truly prevent the mutual diffusion. The disclosed Example uses an interposed layer which contains a component element of the neighboring semiconductor layer, for example, an interposed layer of GaAlAs contains Ga and As, which are the component elements of the neighboring GaAs semiconductor layer. No reasonable grounds are stated as to why the Ga and As of the interposed layer do not diffuse into the Ge semiconductor layer to form a substantial mutual diffusion zone therein.